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Mathlib.Algebra.Category.Ring.Limits

The category of (commutative) rings has all limits #

Further, these limits are preserved by the forgetful functor --- that is, the underlying types are just the limits in the category of types.

The flat sections of a functor into SemiRingCat form a subsemiring of all sections.

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    limit.π (F ⋙ forget SemiRingCat) j as a RingHom.

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      Construction of a limit cone in SemiRingCat. (Internal use only; use the limits API.)

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        Witness that the limit cone in SemiRingCat is a limit cone. (Internal use only; use the limits API.)

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          @[inline, reducible]
          abbrev CommSemiRingCatMax :
          Type ((max u1 u2) + 1)

          An alias for CommSemiring.{max u v}, to deal with unification issues.

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            The chosen cone is a limit cone. (Generally, you'll just want to use limit.cone F.)

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              The forgetful functor from rings to types preserves all limits. (That is, the underlying types could have been computed instead as limits in the category of types.)

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              @[inline, reducible]
              abbrev RingCatMax :
              Type ((max u1 u2) + 1)

              An alias for RingCat.{max u v}, to deal around unification issues.

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                def RingCat.sectionsSubring {J : Type v} [CategoryTheory.SmallCategory J] (F : CategoryTheory.Functor J RingCatMax) :
                Subring ((j : J) → (F.toPrefunctor.obj j))

                The flat sections of a functor into RingCat form a subring of all sections.

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                  We show that the forgetful functor CommRingCatRingCat creates limits.

                  All we need to do is notice that the limit point has a Ring instance available, and then reuse the existing limit.

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                  The forgetful functor from rings to types preserves all limits. (That is, the underlying types could have been computed instead as limits in the category of types.)

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                  @[inline, reducible]
                  abbrev CommRingCatMax :
                  Type ((max u1 u2) + 1)

                  An alias for CommRingCat.{max u v}, to deal around unification issues.

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                    We show that the forgetful functor CommRingCatRingCat creates limits.

                    All we need to do is notice that the limit point has a CommRing instance available, and then reuse the existing limit.

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                    The forgetful functor from commutative rings to rings preserves all limits. (That is, the underlying rings could have been computed instead as limits in the category of rings.)

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                    The forgetful functor from commutative rings to commutative semirings preserves all limits. (That is, the underlying commutative semirings could have been computed instead as limits in the category of commutative semirings.)

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                    The forgetful functor from commutative rings to types preserves all limits. (That is, the underlying types could have been computed instead as limits in the category of types.)

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